The purpose of this program project is to study the processes of regulation and exchange in the microcirculation. The function of the microcirculation is to facilitate the exchange of various substances and regulate the flow of blood in a manner appropriate to the functions of the tissue. Much of the present knowledge of these functions is based upon whole organ studies. The approach used in our studies is to examine the behavior of individual units of the microcirculation, arterioles, capillaries, and venules and organized units such as the arteriolar and capillary networks. In addition, the functions of localized tissue areas adjacent to these vessels is being studied. From this approach we are developing a detailed understanding of the characteristic behavior of the units which comprise the microcirculation. These studies will lead ultimately to a synthesis of behavior of a complete microcirculatory bed. The approach used in these studies is both experimental and theoretical. A number of disciplines are represented, and the contribution of each is essential to effective study of the various processes involved. In this project, we have brought together a group representing the fields of physiology, engineering, and applied mathematics. Their efforts are directed jointly toward the solution of certain key problems which we have identified as important and potentially soluable with existing techniques, or methodology which we propose to develop. While the emphasis of the program project is on advancement of scientific understanding, we recognize that scientific advances are often dependent upon technological developments. Therefore, a portion of the total activities of the program are directed toward development of new instrumentation and improvement of existing systems.